net: dsa: make dsa_slave_dev_check use const

The switchdev handle helpers make use of a device checking helper
requiring a const net_device. Make dsa_slave_dev_check compliant
to this.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>